A method and circuit for providing a controlled ramping and filtering to a
reference voltage in a regulator circuit such that an inrush current is
limited and noise coupling to an output voltage is reduced. In a typical
regulator circuit a soft-start and filter circuit is inserted between a
feedback amplifier and a charge transfer circuit. The soft-start and
filter circuit comprises a pair of transistors that are arranged to
operate as a current mirror, a biasing current source determining the
current drawn from the current mirror, and a soft-start capacitor shunted
by a third transistor providing a conditioned reference voltage. When
reference voltage rises rapidly during power-up, charging of the
soft-start capacitor with constant current provides a soft-sloped
conditioned reference voltage. In steady state, soft-start capacitor and
a resistance of the transistor provide a first order low pass filter
reducing noise that couples to the reference voltage.